PROJECT SUMMARY A critical feature of episodic memory formation is the ability to associate temporally segregated events as an episode, called temporal association learning. Malfunctions of temporal association learning represent well- described findings in human patients suffering from schizophrenia and Alzheimer's disease. There are several critical gaps in our knowledge of current theoretical model and neurobiological evidence about mechanisms of temporal association learning. My long-term goal is to elucidate the neural mechanisms that drive and regulate temporal association learning by understanding neural circuits and their neural processes in entorhinal cortical-hippocampal (EC-HPC) networks, using Pavlovian trace fear conditioning (TFC) as the behavioral paradigm. We previously demonstrated that pOxr1+ excitatory cells in the medial entorhinal cortex layer III (pOxr1+ cells) project to the hippocampal CA1 pyramidal cells and are necessary for TFC. On the other hand, some CalB+ excitatory cells in MECII (CalB+ cells) project to GABAergic neurons in hippocampal CA1, suppress the MECIII input into the CA1 pyramidal cells through the feed-forward inhibition, and inhibit TFC. These findings lead us to propose a disinhibition model to regulate TFC, driving TFC by pOxr1+ cells and regulating TFC by CalB+ cells. The central hypothesis of this model is that successful TFC depends on learning-dependent disinhibition of hippocampal CA1 pyramidal cells through the reduction of feed-forward inhibition mediated by CalB+ cells. Towards this hypothesis, we have identified that pOxr1+ cells show tone-induced sustained neural activity in all trials during TFC, while CalB+ cells show trial-dependent reduction of the tone-induced sustained neural activity. We have also discovered that the CalB+ cells specifically express dopamine D1 receptors (D1R) in MEC and the activation D1R in MEC is essential for the learning-dependent reduction of the c-Fos expression in CalB+ cells and for successful TFC. Guided by strong preliminary data, we propose to pursue three Specific Aims to examine neural circuit mechanism that drive and regulate TFC: (1) To define the roles of pOxr1+ cells and CalB+ cells for TFC. (2) To determine the role of D1R activation in CalB+ cells for TFC. (3) To elucidate the role of dopaminergic inputs into the MEC for TFC. Collectively, our proposed research will broadly impact the field of learning and memory by characterizing novel neural circuits and their neural process that drive and regulate temporal association memory in EC-HPC networks. Our proposed studies will uncover neural substrates for temporal association memory and novel learning-dependent gatekeeper circuits for the regulation of temporal association learning, and potentially, the circuit mechanism can be a pharmaceutical new target for preventing inadequate memory formation.